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모래 함유량이 점토의 액소성한계 및 전단강도에 미치는 영향
박성식,농쩐쩐 한국지반공학회 2014 한국지반공학회논문집 Vol.30 No.2
해안지역의 연약지반을 개량하기 위하여 연약지반 위에 모래를 포설하거나 모래다짐말뚝을 시공할 경우 점토와 모래가 서로 섞이는 경우가 많으며, 풍화된 사면의 표층에도 모래나 자갈 섞인 세립토가 많이 존재한다. 본 연구에서는 이와 같은 혼합토에 포함된 모래 함유량의 증가에 따른 점토의 액소성한계 및 전단강도 변화에 대해 연구하였다.먼저 카올리나이트와 벤토나이트에 모래 함유량을 0, 9, 17, 23, 29, 33, 또는 50%까지 증가시키면서 액소성한계시험을실시하였으며, 모래 함유량이 증가함에 따라 액소성한계는 감소하는 경향을 보였다. 다양한 모래 함유량을 가진 카올리나이트와 벤토나이트에 함수비를 10% 또는 20%씩 단계적으로 증가시키면서 토베인시험기를 이용하여 비배수전단 강도를 측정하였다. 동일한 모래 함유량을 가진 공시체의 경우 특정 함수비에서 비배수전단강도가 급격히 감소하였으며, 공시체 내 모래 함유량이 증가할수록 배수가 발생하면서 비배수전단강도는 감소하였다. 한편 동일한 조건의 혼합토에 대한 직접전단시험을 실시하여 모래 함유량의 증가에 따른 점토-모래 혼합토의 점착력 및 내부마찰각을 측정하였다. 점토 내 모래 함유량이 증가함에 따라 점착력은 감소하였으며, 내부마찰각은 증가하는 경향을 보였다.
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흙의 비배수전단강도가 0이 되는 함수비인 흐름한계의 제안
박성식,농쩐쩐 한국지반공학회 2013 한국지반공학회논문집 Vol.29 No.11
When a slope failure or a debris flow occurs, a shear strength on failure plane becomes nearly zero and soil beginsto flow like a non-cohesive liquid. A consistency of cohesive soils changes as a water content increases. Even a cohesivesoil existing at liquid limit state has a small amount of shear strength. In this study, a water content, at which a shearstrength of cohesive soils is zero and then cohesive soils will start to flow, was proposed. Three types of clays (kaolinite,bentonite and kaolinite (50%)+bentonite (50%)) were mixed with three different solutions (distilled water, sea waterand microbial solution) at liquid limit state and then their water contents were increased step by step. Then, theirundrained shear strength was measured using a portable vane shear device called Torvane. The ranges of undrainedshear strength at liquid and plastic limits are 3.6-9.2 kPa and 24-45 kPa, respectively. On the other hand, the watercontent that corresponds to the value of the undrained shear strength changing most rapidly is called flow water content. The flow limit refers to the water content when undrained shear strength of cohesive soils is zero. In order to investigatethe relationship between liquid limit and flow limit, the cohesive index was defined as a value of the difference betweenflow limit and liquid limit. The new plasticity index was defined as the value of difference between flow limit andplastic limit. The new liquidity index was also defined using flow limit. The values of flow limit are 1.5-2 times higherthan those of liquid limit. At the same time, the values of new plasticity index are 2-5.5 times higher than those oforiginal plasticity index.
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박성식,농쩐쩐,정승원 한국지반공학회 2012 한국지반공학회논문집 Vol.28 No.10
Small amount of salt and various microorganisms are contained in natural soils or clays near the shore. In this study,most common clay minerals such as kaolinite and bentonite are used to evaluate the effect of salt and microorganism on liquid and plastic limits. The effect of time after mixing clays with water was also investigated for the consistency limits of clays. The test was conducted immediately, 1 day, or 7 days after mixing soils. For liquid limit tests two different test methods were used, the percussion cup method by Casagrande and the fall cone method. When sea water or microbial solution was used for mixing soils rather than distilled water, the liquid limit of kaolinite decreased by 6-15% and that of bentonite further decreased up to 37-53%. The liquid limit obtained from the fall cone method was approximately 10% for bentonite and 20% for kaolinite, which are higher than those from the percussion cup method. The effect of time on liquid and plastic limits was significant for those tested immediately or 1 day later but the values obtained after 1 day or 7 days did not varg a lot, regardless of soils, mixing waters or test methods.
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박성식,농쩐쩐,최선규,문홍득 한국지반공학회 2018 한국지반공학회논문집 Vol.34 No.9
A magnitude 5.4 earthquake struck the city of Pohang, North Gyeongsang Province, South Korea on November 15, 2017. Many sand volcanoes were observed on paddy fields, parks and roads. This phenomenon was the first to be observed as a sign of soil liquefaction in South Korea. In this study, two different kinds of ejected Pohang sands were collected from a liquefied paddy field. Those sands were reconstituted into loose and dense conditions and then a series of cyclic simple shear tests were conducted under confining stresses of 100 and 200 kPa. A real earthquake motion was also repetitively applied to the specimen. As a result of constant shear stress tests, the cyclic resistance ratio (CRR) of loose sand was 0.12-0.14, while the CRR value of dense sand was 0.17-0.21. It was shown that the relative density was more influencing factor on liquefaction resistance than the sand types and initial confining stress. When a real Pohang earthquake motion was repetitively applied to the specimen, a loose sand was liquefied at the second earthquake motion but the dense sand at the third earthquake motion.
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